Clean water supplies can become contaminated by hazardous connections to the piping system known as cross-connection. A cross-connection is a connection between a drinking water plumbing system and any other source that may contribute to the degradation of the quality of drinking water.
A backflow preventer is typically used to prevent contamination of a main water supply line by reducing or eliminating backflow of contaminated water into the supply line. A backflow preventer is installed in, for example, a pipeline between a main water supply line and a service line and designed to protect, for example, drinking water plumbing systems in accordance with national plumbing codes for non-health hazard and health hazard cross-connections and continuous pressure applications.
A conventional backflow preventer may include a single check valve unit or a pair of check valves arranged serially within a valve housing. A pair of check valves is commonly used in the event one of the valves fails. The housing includes a first port connected to the main water supply line and a second port connected to, for example, a service line leading to industrial and commercial water users. These preventers stop any reverse flow of liquid pollutants from those facilities into the main water supply system. In one application, a backflow preventer is installed in a fire sprinkler system to prevent backflow and subsequent cross contamination of old water that may exist in the fire sprinkler system from entering the main water supply system. Another typical installation is in the water supply line and fire lines to a hospital or an industrial facility.
During operation, water flow pressure urges the check valves into an open position, which allows water to flow freely through the valve. The check valves are normally spring loaded and biased into a closed position under no or minimal pressure and a backflow of liquid from the second port to the first port is prevented.
The pressure required to open a backflow preventer using check valves utilized in drinking water supply lines is established by regulation and provides a minimum degree of protection against reverse backflow. For example, backflow check valves used for fire-protection service are regulated by standards set by the Underwriters Laboratories Inc. and American National Standards Institute (ANSI). As one example, a double or single detector backflow check valve used in a fire sprinkler system must be design to meet the ANSI/UL 312-2003 Standard For Check Valves for Fire-Protection Services, which is hereby incorporated herein by reference. In use in a fire sprinkler system, the check valves are designed not to open (e.g. crack-open) until the pressure difference is one pound per square inch (1 PSI) across the check valve.
A potable water supply system must be designed, installed and maintained in such a manner so as to prevent contamination from nonpotable liquids, solids or gases. For use in drinking water supply lines, the backflow preventers should conform to the standards developed and set forth by the American Society of Sanitary Engineers (ASSE) and the Foundation for Cross-connection and Hydraulic Research (FCCHR), and the School of Engineering at the University of California. In use in drinking water supply lines, the first check valve in a backflow preventer using two check valves in series, are required not to open (e.g. crack-open) until the pressure differential is three pounds per square inch (3 PSI) across the check valve and at a minimum of one pound per square inch (1PSI) for the second check valve.
Conventional backflow preventers use check valves having a linear relationship between friction loss and the amount of flow. Specifically, a conventional check valve will open under a linear relationship between a minimal amount of pressure and a minimal amount of flow. However, when used in, for example, a domestic water supply, there are concerns about leaks and water theft.
In an effort to detect such events, backflow preventers are constructed to resist opening under a minimal amount of pressure and a bypass line is installed incorporating a water meter. When forward water flow produces only small forces (e.g. minimal amount of pressure) insufficient to separate the check valve from its valve seat, the water is directed through the meter in the bypass line and the check valve does not open. Current construction of these types of check valves, however, is complicated and expensive.